Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Discrete clouds of neutral gas between the galaxies M31 and M33


Spiral galaxies must acquire gas to maintain their observed level of star formation beyond the next few billion years1. A source of this material may be the gas that resides between galaxies, but our understanding of the state and distribution of this gas is incomplete2. Radio observations3 of the Local Group of galaxies have revealed hydrogen gas extending from the disk of the galaxy M31 at least halfway to M33. This feature has been interpreted to be the neutral component of a condensing intergalactic filament4, which would be able to fuel star formation in M31 and M33, but simulations suggest that such a feature could also result from an interaction between both galaxies within the past few billion years (ref. 5). Here we report radio observations showing that about 50 per cent of this gas is composed of clouds, with the rest distributed in an extended, diffuse component. The clouds have velocities comparable to those of M31 and M33, and have properties suggesting that they are unrelated to other Local Group objects. We conclude that the clouds are likely to be transient condensations of gas embedded in an intergalactic filament and are therefore a potential source of fuel for future star formation in M31 and M33.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: H i column density maps.
Figure 2: A position–velocity plot of the H i clouds and selected Local Group objects.
Figure 3: A plot of baryonic mass versus the equivalent circular rotation speed.


  1. 1

    Leroy, A. K. et al. The star formation efficiency in nearby galaxies: measuring where gas forms stars effectively. Astron. J. 136, 2782–2845 (2008)

    CAS  Article  ADS  Google Scholar 

  2. 2

    Shull, J. M., Smith, B. D. & Danforth, C. W. The baryon census in a multiphase intergalactic medium: 30% of the baryons may still be missing. Astrophys. J. 759, 23–37 (2012)

    Article  ADS  Google Scholar 

  3. 3

    Braun, R. & Thilker, D. A. The WSRT wide-field H I survey II. Local Group features. Astron. Astrophys. 417, 421–435 (2004)

    Article  ADS  Google Scholar 

  4. 4

    Davé, R. et al. Baryons in the warm-hot intergalactic medium. Astrophys. J. 552, 473–483 (2001)

    Article  ADS  Google Scholar 

  5. 5

    Bekki, K. Formation of a giant HI bridge between M31 and M33 from their tidal interaction. Mon. Not. R. Astron. Soc. 390, L24–L28 (2008)

    Article  ADS  Google Scholar 

  6. 6

    Lockman, F. J., Free, N. L. & Shields, J. C. The neutral hydrogen bridge between M31 and M33. Astron. J. 144, 52–58 (2012)

    Article  ADS  Google Scholar 

  7. 7

    Popping, A., Davé, R., Braun, R. & Oppenheimer, B. D. The simulated H I sky at low redshift. Astron. Astrophys. 504, 15–32 (2009)

    CAS  Article  ADS  Google Scholar 

  8. 8

    Maloney, P. Sharp edges to neutral hydrogen disks in galaxies and the extragalactic radiation field. Astrophys. J. 414, 41–56 (1993)

    CAS  Article  ADS  Google Scholar 

  9. 9

    Ibata, R. et al. The haunted halos of Andromeda and Triangulum: a panorama of galaxy formation in action. Astrophys. J. 671, 1591–1623 (2007)

    CAS  Article  ADS  Google Scholar 

  10. 10

    McConnachie, A. W. et al. The remnants of galaxy formation from a panoramic survey of the region around M31. Nature 461, 66–69 (2009)

    CAS  Article  ADS  Google Scholar 

  11. 11

    Lewis, G. F. et al. PAndAS in the mist: the stellar and gaseous mass within the halos of M31 and M33. Astrophys. J. 763, 4–13 (2013)

    Article  ADS  Google Scholar 

  12. 12

    Tully, R. B. & Fisher, J. R. A new method of determining distances to galaxies. Astron. Astrophys. 54, 661–673 (1977)

    ADS  Google Scholar 

  13. 13

    McGaugh, S. S., Schombert, J. M., Bothun, G. D. & de Blok, W. J. G. The baryonic Tully-Fisher relation. Astrophys. J. 533, L99–L102 (2000)

    CAS  Article  ADS  Google Scholar 

  14. 14

    Fukugita, M. & Peebles, P. J. E. The cosmic energy inventory. Astrophys. J. 616, 643–668 (2004)

    CAS  Article  ADS  Google Scholar 

  15. 15

    Gentile, G. et al. Tidal dwarf galaxies as a test of fundamental physics. Astron. Astrophys. 472, L25–L28 (2007)

    Article  ADS  Google Scholar 

  16. 16

    Maller, A. H. & Bullock, J. S. Multiphase galaxy formation: high-velocity clouds and the missing baryon problem. Mon. Not. R. Astron. Soc. 355, 694–712 (2004)

    CAS  Article  ADS  Google Scholar 

  17. 17

    Fernández, X., Joung, M. R. & Putman, M. E. The origin and distribution of cold gas in the halo of a Milky-Way-mass galaxy. Astrophys. J. 749, 181–192 (2012)

    Article  ADS  Google Scholar 

  18. 18

    Chynoweth, K. et al. Neutral hydrogen clouds in the M81/M82 group. Astron. J. 135, 1983–1992 (2008)

    CAS  Article  ADS  Google Scholar 

  19. 19

    Boothroyd, A. I. et al. Accurate galactic 21-cm H I measurements with the NRAO Green Bank Telescope. Astron. Astrophys. 536, A81 (2011)

    Article  Google Scholar 

  20. 20

    Thilker, D. A. et al. On the continuing formation of the Andromeda galaxy: detection of H I clouds in the M31 halo. Astrophys. J. 601, L39–L42 (2004)

    CAS  Article  ADS  Google Scholar 

  21. 21

    Grossi, M. et al. H I clouds in the proximity of M33. Astron. Astrophys. 487, 161–175 (2008)

    CAS  Article  ADS  Google Scholar 

  22. 22

    Westmeier, T., Brüns, C. & Kerp, J. Relics of structure formation: extra-planar gas and high velocity clouds around the Andromeda Galaxy. Mon. Not. R. Astron. Soc. 390, 1691–1709 (2008)

    CAS  ADS  Google Scholar 

  23. 23

    Putman, M. E. et al. The disruption and fueling of M33. Astrophys. J. 703, 1486–1501 (2009)

    CAS  Article  ADS  Google Scholar 

  24. 24

    McGaugh, S. S. & Wolf, J. Local Group dwarf spheroidals: correlated deviations from the baryonic Tully-Fisher relation. Astrophys. J. 722, 248–261 (2010)

    Article  ADS  Google Scholar 

  25. 25

    Tollerud, E. J. The SPLASH survey: spectroscopy of 15 M31 dwarf spheroidal satellite galaxies. Astrophys. J. 752, 45–73 (2012)

    Article  ADS  Google Scholar 

  26. 26

    McGaugh, S. S. The baryonic Tully-Fisher relation of gas-rich galaxies as a test of ΛCDM and MOND. Astron. J. 143, 40–54 (2012)

    Article  ADS  Google Scholar 

Download references


D.J.P. is an Adjunct Assistant Astronomer at the National Radio Astronomy Observatory; F.J.L. is an Adjunct Professor in the Department of Physics, West Virginia University. S.A.W. acknowledges the student observing support (GSSP11-012) provided by the NRAO for this project. The NRAO is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc. D.J.P. acknowledges support from NSF CAREER grant AST-1149491. S.S.M. acknowledges support in part by NSF grant AST-0908370.

Author information




All authors assisted in the development and writing of this work. S.A.W. is the Principal Investigator, and was responsible for data collection, processing and analysis. D.J.P. and F.J.L. assisted in the experimental design, data collection and analysis. S.S.M. provided the techniques of estimating the baryonic masses. E.J.S. provided necessary insights on potential interactions with M31 and ways of estimating circular rotation speeds. All authors aided in the interpretation of the results and provided comments for revisions to this work.

Corresponding author

Correspondence to Spencer A. Wolfe.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Figure

This file contains Supplementary Figure 1, which presents channels maps, as a single figure, to allow the reader to see the HI clouds' evolution through the data cube. The text points out the velocity ranges of each HI cloud and points out specific features, when apparent. (PDF 1495 kb)

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Wolfe, S., Pisano, D., Lockman, F. et al. Discrete clouds of neutral gas between the galaxies M31 and M33. Nature 497, 224–226 (2013).

Download citation

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing